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Early in the 1950's it was first discovered that type O seemed to predominate over the other types in the occurrence of all types of stomach ulcers at a rate of approximately two to one. These findings have been reproduced so many times (over 25 studies in the last 20 years alone) with the same consistent result (type O getting more ulcers than the other types) that the conclusions are virtually unquestionable.

The same time that blood type O was shown to be more prone to ulcers, it was discovered that pernicious anemia, a disorder associated with low stomach acid, was linked to blood type A.

In 1959, the production of both stomach acid and pepsin was studied in both Blacks and Caucasians and analyzed according to ABO blood type. While the secretion of both gastric acid and pepsin was about the same for both races, substantial differences were found between the blood types. Impairment of stomach acid production was much more common in type A than type O, and the value of plasma pepsinogen was also higher in type O over type A.

In a separate study, the same researcher also found that a lack of stomach acid was significantly more common among group A individuals reporting digestive problems. Other studies done around the same time in India also showed differences in stomach acid production between type O and type A in response to a standardized test meal. One reason that low stomach acid is linked to type A blood may have to do with the role of Epidermal Growth Factor (EGF) which we discussed in the previous section on saliva. As we discovered, the receptor for EGF has a high affinity for the type A antigen, and though the major action of EGF is to stimulate repair of the digestive lining, it has also been shown to decrease acid production in the stomach.

Most protein in the diet is derived from animal sources, though vegetables do contain smaller amounts. As we have seen, type O manufactures more hydrochloric acid in their stomachs than the other types, and with pepsin, pepsinogen and gastrin, the same is true.

In one study, serum pepsinogen levels were studied in relation to ABO blood group, age and sex in 700 healthy blood donors. Serum pepsinogen levels were higher in males than in females and rose with increasing age. Blood group O individuals showed higher serum pepsinogen levels compared with blood group A. There is also evidence that the type A antigen in gastric juice binds to pepsin and possibly inactivates it. In a separate study, the concentrations of gastrin were measured in the blood of 121 fasting healthy Greek volunteers of both sexes and of different ABO blood types, aged between 20 and 70 years. The testing took place after a test meal while the subjects were fasting, and again at 10 minutes and 40 minutes. The researchers found that gastrin levels took 40 minutes to increase after the meal in the blood types A and B while in the blood type O significant increase had appeared already 10 min after the meal.

These studies conclusively show that ABO blood type has a clear effect on the characteristics of one's stomach activity. Acidity, pepsin, pepsinogen and gastrin; all needed for the digestion of concentrated proteins, all found in greater abundance in type O over the other types. Perhaps one association could be considered an anomaly; this clearly shows something much different.

In addition to digesting proteins, the acid in the stomach serves are a barrier to most bacteria. This is an important function, because the mixture of food swallowed from the mouth and mixed with saliva is not sterile, and since the upper small intestine is designed for absorption it would not be desirable for there to be large amounts of bacteria in this part of the gut. One of the major problems with having low stomach acidity (a condition called hypochlorhydria ) is an excess bacterial overgrowth in the stomach and in the upper small intestine. This bacteria growth tends to be a chronic problem which recurs within days or weeks after antibiotics are discontinued.